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On the Geometric Nature of Low-frequency Quasi-periodic Oscillations in Neutron-star Low-mass X-Ray Binaries

Journal

Astrophysical Journal

Volume | Issue number

812 | 1

Article number

80

Number of pages

8

Document type

Article

Faculty

Faculty of Science (FNWI)

Institute

Anton Pannekoek Institute for Astronomy (API)

Abstract

We report on a detailed analysis of the so-called ~1 Hz quasi-periodic oscillation (QPO) in the eclipsing and dipping neutron-star
low-mass X-ray binary EXO 0748-676. This type of QPO has previously been shown to have a geometric origin. Our study focuses
on the evolution of the QPO as the source moves through the color-color diagram in which it traces out an atoll-source-like
track. The QPO frequency increases from ~0.4 Hz in the hard state to ~25 Hz as the source approaches the soft state. Combining
power spectra based on QPO frequency reveals additional features that strongly resemble those seen in non-dipping/eclipsing
atoll sources. We show that the low-frequency QPOs in atoll sources and the ~1 Hz QPO in EXO 0748-676 follow similar relations
with respect to the noise components in their power spectra. We conclude that the frequencies of both types of QPOs are likely
set by (the same) precession of a misaligned inner accretion disk. For high-inclination systems like EXO 0748-676 this results
in modulations of the neutron-star emission due to obscuration or scattering, while for lower-inclination systems the modulations
likely arise from relativistic Doppler-boosting and light-bending effects.

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